The invention relates to a method of producing a sintered dental prosthesis with a metallic structural matrix consisting of a noble-metal powder mixture or noble-metal alloy powder mixture with bi or multimodal particle size distribution and particles being primarily spherical in shape. The powder mixture is stirred with a mixing or conditioning liquid consisting basically of water to form a suspension capable of being modelled or molded as well as compressed so as to expel the mixing liquid. The dental prosthesis is formed by molding the suspension onto a model. The model functions as a firing carrier for the suspension molded onto the model through way of a technique customary in dental ceramics. The molded suspension is subsequently sintered onto the model in a graphite box or under a protective gas.
The production of a metallic dental prosthesis is often used for the prosthetic replacement following loss of teeth due to any one of the various dental diseases or after an accidental loss of one or more teeth. Examples of such prosthetic replacements include inlays, crowns and bridges which can be veneered with ceramics or plastic or which, when not veneered, can be cast with the so-called "wax-melt-out method", a casting technique which assures high dimensional accuracy.
The advantages of producing crowns and bridges with the "wax-melt-out method" include the aforementioned dimensional accuracy, high strength and a desirable degree of ductility, which must be assured in the case of rather large bridge constructions in order to avoid forced ruptures upon overloading. On the other hand, the method itself is very time-consuming, material-intensive and equipment-intensive. The necessity of using runners and casting cones causes a use of material which is distinctly elevated in relation to the weight of the cast object and which can result, in the case of repeated usage, in changes of the alloy properties and, if it is not reused, remains as scrap. Another disadvantage of this technique is the fact that in the case of errors in the cast object, a repair is not possible but rather the entire production process, starting with the wax modelling, must be repeated.
DE-OS No. 1 915 977 describes a method for the production of a metallic dental prosthesis using sintering technology in which the prosthesis is produced by applying to a model of the teeth a paste consisting of metal powder with a particle size between 2 and 25 .mu.m and of a binder functioning as an adhesive and subsequently sintering the molded paste. One disadvantage PG,4 of such a method resides in the poor compressibility of the described pastes, since the binder acting as adhesive cannot be expelled by compressing methods such as grooving or vibration. Moreover, since a powder fraction is used at the start, the density of the green compact is low, resulting in a great deal of shrinkage during sintering. Consequently, a high accuracy of fit is unobtainable which is not tolerable for most applications. The use of very fine powders between 2 and 25 .mu.m does assure a very high sintering activity but entails high production costs in addition.
U.S. Pat. No. 4,661,071 discloses a method for the production of metallic dental prostheses using sintering technology and making use of powder having a size between 5-90 .mu.m. The powder is made into a paste with a suitable binder and the metallic dental prosthesis is modelled on a model of the teeth to be provided. A special, castable and self-hardening edgeless mass is necessary for the production of the model which must be burned before the application of the metal powder at 1400.degree. C.-1460.degree. C.
Since traditional dental-ceramic firing-on kilns achieve maximum temperatures of up to approximately 1200.degree. C., a special kiln is necessary for the process of U.S. Pat. No. 4,661,071. A liquid phase sintering process is used for sintering the metal powder under a vacuum of 1 HPa to 10.sup.-2 HPa. Since traditional dental-ceramic firing-on kilns does not achieve this vacuum, a special vacuum kiln is likewise necessary. Kilns with maximum temperatures of up to 1400.degree. C. and kilns which assure a good vacuum at high temperatures are very much more expensive than normal ceramic firing-on kilns, so that the use of this method requires an expensive investment in equipment for a dental technician. Moreover, the use of the liquid phase sintering process results in problems in form stability during the sintering. In order to obtain as rapid a compression as possible by means of a rearrangement of the solid components, a liquid phase portion of at least 30-35% is necessary. (R. M. German, Liquid Phase Sintering, Plenum Press, N.Y., pp. 4,6,80). In analogy with the behavior of dental firing-on ceramics, a rounding or flattening of very delicate details, e.g. of an occlusal surface, must be reckoned with which can result in problems regarding the contact points and can possibly require considerable reworking.
DE-OS No. 35 32 331 describes a method for the production of metallic protheses using sintering technology which achieves a purposefully high density of the green compact using a powder mixture with multimodal size distribution. The powder mixture is converted with water into a suspension which is capable of being modeled and compressed. Accordingly, the shrinkage during sintering remains small. This is advantageous for the obtention of a good fit accuracy. The use of water as a mixing liquid to provide a consistency which is very similar to that of dental veneer ceramic suspensions permits an additional compression by means of expelling the liquid with the technology customary in dental ceramics (grooving, etc.). The sintering process can be carried out without very great expense in a traditional dental ceramic firing-on kiln. This can be achieved on the one hand by means of using a graphite box in which the modelled dental prosthesis to be sintered is located. This graphite box is placed in a customary dental ceramic firing-on kiln and assures a protection against the oxidation of base-metal components of the alloy at the sintering temperature. On the other hand, the introduction of protective gas into the ceramic firing-on kiln can likewise achieve a sufficient reduction of the partial oxygen pressure. After the sintering, the dental prosthesis is cooled off in the graphite box in air.
It proved to be disadvantageous in the above method, when using powder mixtures consisting of atomized, primarily spherical noble-metal alloys and precipitated, very fine, primarily spherical noble-metal powders, that maximum density values could not be achieved in the sintered state with the sintering parameters described in the method of DE-OS No. 35 32 331. This disadvantage is especially problematic in the case of multiple sintering as the density of the sintered prosthesis drops distinctly. However, multiple sintering can be necessary when producing bridges in several work steps or in the case of edge corrections.